There are many instances in the use of telescopes in which it is necessary or desirable to rotate the telescope about its polar axis. For example, because of the rotation of the earth, an astronomical telescope must be rotated about its polar axis at a constant rate of one revolution per day when making long time studies and photographs of cellestial bodies.
A variety of mounting systems for astronomical telescopes have been devised. One of the most common astronomical telescope mounting systems is known as the equatorial mount. The equatorial mount embodies a two-axes gimbal system having an inclined rotation axis which parallels the earth's rotational axis, and having a second horizontal rotational axis which intersects the inclined axis at right angles to the vertical plane containing the latter axis. The inclined axis is referred to as the polar axis of the telescope, and the perpendicular axis is referred to as the declination axis. When the polar axis is adjusted to be parallel to the earth's axis, the elevation angle of the telescope will correspond to the latitude at which the telescope is used. Rotation of the mounting unit about the polar axis moves the telescope in a rotary tracking motion as explained above.